Static electricity is the phenomenon of electric charge at rest, and is generally produced by friction or electrostatic induction.
The buildup of static electricity occurs through a process in which charge transfer takes place between dissimilar materials, at least one of which has a high electrical resistivity. Static electricity often occurs due to rubbing or mere contact between these materials. Such contact and separation of materials occurs during a number of industrial and manufacturing operations, including powder processing, and the manufacture of plastic and other materials in certain forms, particularly sheets.
In some circumstances, the buildup of static electricity can produce a significant physical danger. For example, highly insulating synthetic materials such as polymeric powders and insulating liquids tend to charge up readily and large quantities of electrical energy can accumulate. Such large static charges produce a significant risk of incendiary discharge. Under certain circumstances, and depending upon the material being handled, powerful explosions can result. As a specific example, strict grounding procedures are followed during the refueling of aircraft, ships and other large vehicles because of the risk presented by static electricity.
In other industries, static electricity does not provide a risk of significant physical danger, but instead, small amounts of static electricity can damage sensitive parts. For example, in the microelectronics industry, relatively low energy static discharges can damage microelectronic systems or corrupt data maintained on magnetic media.
As another example, when films of insulating material are wound over rollers, such as in the photography industry, surface charging and subsequent discharging can occur and can produce reactions in the photographic emulsion that can damage the film and render it unmarketable.
Accordingly, in industries that handle materials that are sensitive to even small static discharges, precautions must be taken to avoid the detrimental effects the discharges cause. Because as noted above, static electricity tends to build up when certain insulating materials contact one another, one solution is to manufacture as many containers and handling devices as possible out of conductive materials, or to modify contacting surfaces in some fashion which renders them conductive enough to reduce or eliminate static electricity and its accompanying problems.
All static protective materials are by definition conductive to some extent. In order to classify the relative conductivity of widely differing materials in useful fashion, however, the terms "antistatic" and "static-dissipating" (or "electrostatic dissipating" or "dissipative") are also used. For example, the U.S. Department of Defense identifies conductive materials as those with a surface resistivity of 10.sup.5 ohms/square or less, static-dissipating materials as those with a surface resistivity of between 10.sup.5 and 10.sup.9 ohms/square, and antistatic materials as having a surface resistivity of between 10.sup.9 and 10.sup.14 ohms/square (DOD-HDBK-263).
The Electronics Industries Association offers a somewhat different scheme (EIA Standard RS-541) and categorizes materials in only two ranges. Conductive materials are defined as those having a surface resistivity of less than 10.sup.5 ohms/square or a volume resistivity of less than 10.sup.4 ohms-cm. The EIA defines static-dissipative materials as having either a surface resistivity from 10.sup.5 ohms/square to 10.sup.12 ohms/square, or a volume resistivity from 10.sup.4 ohms-cm to 10.sup.11 ohms-cm.
Finally, the International Electrotechnical Commission (IEC) in its publication 801-2 defines antistatic materials as those with a surface resistivity between 10.sup.5 and 10.sup.11 ohms/square.
Accordingly, it will be understood that as used herein and in the industry, terms such as "antistatic," "static-dissipating," and "conductive" are used somewhat interchangeably and represent descriptive classifications rather than absolute or limiting ones.
One method of rendering objects conductive is to coat their surfaces with materials having conductive properties. There are a number of compounds available for such applications. One common group of materials are the quaternary ammonium salts. As well known to those of ordinary skill in this art, however, the conductivity of quaternary ammonium salts is highly dependent upon relative humidity and the salts tend to lose conductivity at low relative humidities. There are, however, a wide range of antistatic compositions available that fall within the broad definition of quaternary ammonium salts.
Another category of antistatic compositions are the derivatives of fatty acids; i.e. carboxylic acids with long hydrocarbon chains. Although useful for antistatic coatings or additives in some situations, the fatty nature and derivation of these compounds makes them unsuitable for many other applications.
Highly conductive materials such as metals or carbon black also provide good static-dissipating properties, but can cause color or opacity problems in many situations, particularly where a transparent surface is required.
As noted above, certain industries and products such as microelectronics and photography, can be sensitive to even small static discharges. Accordingly, packaging of such materials, particularly the packaging of microelectronic components, should preferably exhibit antistatic properties. Because adhesive tape is such a common packaging material, a conductive adhesive tape is a quite useful element in many packaging situations, particularly for microelectronic components.
Therefore, the need exists for a conductive adhesive tape that exhibits the properties necessary to avoid static discharge damage to packaged components, and yet which maintains the transparency and adhesive properties that are fundamental requirements for the packaging tape even apart from its conductive or static-dissipating or antistatic properties.